1. Field of the Invention
This invention relates to an antenna system in a transponder for modulating signals from a reader and for reflecting the modulated signals back to the reader to pass information from the transponder to the reader.
2. Description of Related Art
RFID (radio frequency identification) tags have been used for highway toll collections, tracking railroad freight cars, parking access, and inventory controls. These RFID tags typically consist of an antenna, an antenna impedance matching circuit, and an Application Specific Integrated Circuit (ASIC). The antenna receives the RF signals from the interrogator (reader), and feeds the signal to the ASIC through the antenna matching circuit between the antenna and ASIC. ASIC has hardware and software circuits to handle the RF signals and the signal processing respectively.
In the early 1990s, passive Radio Frequency Identification (RFID) systems were selected by the Association of American Railroads (AAR) for continent-wide electronic identification of railroad rolling stock. Such systems were designed for the harsh rail environment and exhaustively tested by the AAR. Performance, electronic, microwave, and mechanical specifications were selected so that the RFID equipment would not only survive the harsh rail environment, but also have a very long life. Passive tags (i.e. with no battery and using modulated backscatter technology) were installed, two on each rail car, beginning about 1991. The tags were to operate at an electric field strength of 2 V/m rms or higher in the frequency band of 902 MHz to 928 MHz. The tags were to survive incident electric field strength of 50 V/m of continuous exposure for 60 seconds for a radio signal of any frequency including in the design band of 902 MHz to 928 MHz. Mechanical requirements for solar radiation, impact, solvents, etc. were also specified and the tags were designed that would meet the requirements.
Early in 1992, reports of tag failures began to surface. The failures were not wide spread, and appeared to be higher on the west coast. Initially, the damage was thought to be caused by electrostatic discharge (ESD) damage during tag programming. Tags were programmed through physical contact, placing the tag in a programming head of a programmer. Efforts to reduce ESD using industry-approve techniques (wrist straps at programming stations, etc.) failed to reduce the problem.
Next, based on information that the majority of initial tag failures were observed on the west coast, a plan was developed to try and identify where and how the tags were damaged. The damage was known to look like ESD, affecting the sensitive diodes on the tag antenna used to convert RF (radio frequency) signals to DC (direct current) power. No physical damage was observed to the case or circuit board of the tag.
The source of the damage was determined to be a high power (megawatts) air-traffic control radar dish with a high gain antenna (about 40 dB) operating near 1300 MHz. The radar was pulsed, and the dish rotated slowly, scanning for aircraft. The radar was placed close to a railroad and a highway ran there between. When present, large trucks on the highway could protect rail cars from the radar by blocking the line of sight between the radar dish and rail cars. This explains why only a small percentage of tags on the side of the train facing the radar dish were damaged; even though electric field strengths in the area could be enhanced by a phenomenon known as multipath. An engineering investigation and studies indicated that tags passing near the radar dish would need to survive in a pulsed microwave field of 1,500 V/m at 1300 MHz, which is slightly above the targeted 902 MHz to 928 MHz frequency band of the tags.
In the fall of 1992, specifications were set by the AAR and hardening of the tag began. The new specifications (listed in the AAR S918, page K88) were 1,500 V/m pulsed and 100 V/m CW (an increase from the earlier number of 50 V/m). In particular, RFID tags for the railroad application operating at 915 MHz band are required to survive from the radar signal of 1500 V/m field strength at 1,300 MHz. The percentage separation between the operating frequency and the radar signal is only 28% which is too close to filter out a 1300 MHz signal at a negligible cost.
One technical solution that met the requirements used a microwave PIN limited diode between the output of the tag antenna and the input of the matching section such as disclosed in U.S. Pat. No. 4,816,839 to Jeremy Landt. The industry has used a limiter diode and subsequently a discrete component band pass filter to protect the 915 MHz railroad RFID tags from the radar signal of 1500 V/m field strength at 1,300 MHz. Low (i.e. 5 ohms) and high (i.e. 100 ohms) impedance transmission lines are used for the matching circuit between the limiter diode and the voltage doubler, which is a front-end RF circuit of the ASIC. The characteristics of this matching circuit changes with frequency. The discrete component band-pass filter works for the frequency range below the self-resonant frequencies. Therefore both the limiter and discrete filters protect the RFID tags from the high power source within a limited frequency range rather than the entire frequency range.
The effectiveness of the technical solution and the design of the tags are proven by over 15 years of operation of these tags in the harsh rail environment without problems, however, the implementations have added significantly to the cost of the tag. In particular, the use of limiter diodes and discrete band filters adds a considerable cost increase of $0.50 and $0.20, respectively per tag, which is significant considering the large product volume. There has been a long-standing need in the industry to provide protection from high power microwaves, such as radar discussed above, at reduced costs for expansion of the transponders in the market.
All references cited herein are incorporated herein by reference in their entireties.